1. Technical Field
The present invention relates to frequency multipliers and, in particular, to frequency multipliers operating at millimeter wave frequencies.
2. Description of the Related Art
Frequency doublers and frequency multipliers in general are components of millimeter wave (mmWave) data communication, radar and imaging systems. In any semiconductor technology, direct signal generation through an oscillator becomes more difficult as the fundamental frequency increases. High frequency oscillators tend to have lower tuning range, higher phase noise, higher power consumption and decreased robustness against temperature and other environmental variations than oscillators generating low frequencies. For these reasons, to generate a given frequency in the mmWave regime, a common alternative is to employ a robust oscillator at a moderate frequency followed by a frequency multiplier.
Conventional frequency doublers use two transistors driven with a differential signal at a fundamental frequency w and a common load ZL. The current at the second harmonic, having a frequency of 2w, has two components. The first is the second harmonic of the current generated by each transistor. While the current generated at the fundamental frequency has an opposite phase for each transistor and cancels out, the current at the second harmonic has the same phase from each transistor, and hence adds coherently. To enhance this component, transistors are usually biased below their threshold voltage, where their response is non-linear. This enhances harmonic generation but reduces operation speed and saturated output power. The second component comes from the fact that the output voltage swings twice for every cycle of the differential input signal. However, in a conventional frequency doubler, the output impedance at the fundamental frequency is a short, which makes the voltage swing at collector nodes small. This small swing at the collector prevents transistors from generating high power at high efficiency.
Conventional frequency doublers therefore rely on the constructive addition of the two above-described components. However, this constructive addition only occurs for a limited range of input powers. Furthermore, the range and the associated conversion gain are vulnerable to device and temperature variations, which affect the threshold voltage and hence the device's non-linearity.